The well. It is a collapsed sinkhole formed when
underground waterflows eroded the overlying
strata, forming a cavity. When the cavity grew
big enough to not be able to support the top,
it fell down, leaving a water-filled hole in the
middle of the desert.

The hole seen from the opposite rim. Note the houses
built into the wall - it was a popular way
among the people living here of building houses that
were well-protected and warm.